It is conventional to provide metal detectors in crop processing machines such as forage harvesters, the purpose of the metal detectors being to detect metal picked up from a field with the crop material. When the metal detector detects the passage of a metal object through the crop feed path, the detector produces an output signal to stop the crop feed mechanism before the metal object can be fed into the crop cutter knives.
As illustrated in Bohman U.S. Pat. No. 4,433,528, the metal detector detection coils and associated detection circuits may be located within a housing to protect them from dust and moisture and the housing may in turn be located within a rotatable crop feed roll so as to position the detection coils as close as possible to the crop feed path. Furthermore, the coils may also be encased in a potting material. This makes it difficult to access the detection coils for test purposes. On the other hand, it is desirable that the metal detector be frequently checked to assure its sensitivity and operability because if the metal detector is not functioning properly metal objects may not be detected and thus may enter the cutter mechanism where they may cause considerable damage.
It would be possible to provide an additional coil for testing purposes, the test coil being disposed in proximity to the detection coils so that an emf (electromotive force) is induced in the detection coils upon application of a signal to the test coil. However, this adds to the cost of producing the system. Furthermore, the limited space available within the housing in which the metal detector is located makes it difficult to add an additional coil.
Some metal detectors, such as the one illustrated in U.S. Pat. No. 4,433,528 have two channels. That is, the detectors have two detection coils each connected to a respective one of two detection circuits which serve to amplify and filter the output signals from the detection coils. To insure a more uniform sensitivity of the metal detector across the width of the crop feed path, each detection coil is formed so as to comprise a series of generally triangular segments, the segments of one coil nesting between two segments of the other coil. Because of the close spacing, the detection coils are mutually inductively coupled. The present invention utilizes this mutual inductance in the testing of the inductance of the coils.